BELGRADE – Two U.S.-bound missiles found on a passenger flight from Lebanon to Serbia were used for training and did not contain any explosives, the Lebanese army said Monday. Serbian investigators remain unconvinced, saying police found traces of suspected explosive materials on the cargo.

The Lebanese army said the missiles were being sent from Beirut to the American company that produced them. It said the return was in accordance “with administrative and legal measures after the training ended.”

But the Serbian public prosecutors’ office said Monday in a statement emailed to The Associated Press it was investigating findings by border police that the packages “are suspected to contain explosive materials.” It said the investigation also focused on where the missiles came from and what was their final destination.

“We hope that our expertise will be finished this week,” prosecution spokesman Ivan Markovic said, adding that the final report could take time “because some of the information is located in foreign countries.”

Serbia’s N1 television has said the packages, with two guided armor-piercing missiles, were discovered Saturday by a sniffer dog after an Air Serbia flight landed at Belgrade airport from the Lebanese capital, Beirut. Serbian media said documents listed the final destination for the AGM-114 Hellfire missiles as Portland, Oregon, with a stopover in London.

The American-made projectiles can be fired from air, sea or ground platforms. They are also launched from drones in the U.S. fight against terrorism. The practice version of Hellfire is fitted with an inert warhead without explosives.

A U.S. official familiar with the situation, speaking on condition of anonymity because he wasn’t authorized to discuss the case, said the missiles are “inert training dummies.”

“There was no explosive residue, no warhead and no engine,” he said.

These are formally called “captive air training missiles” and they had been sent to Lebanon by their manufacturer, Lockheed Martin, for the Lebanese armed forces to practice mounting them on their Cessna planes, according to the official.

He said it is not clear how the Serbs picked up explosive residue on the crates, but added it was possible that it was a mistake or that someone who loaded them in Lebanon may have gotten some residue on them.

The Lebanese Al-Joumhouria newspaper, which is owned by former defense minister Elias Murr, reported that the Hellfire missiles were delivered to Lebanon by the U.S. in October 2015, adding that they are laser-guided and specialized for training. It said the missiles were being shipped backed to the U.S. after training ended in Lebanon, with the knowledge of Lebanese and American authorities.

The head of Beirut’s Rafik Hariri Airport issued a statement saying that the missiles do not contain any explosives.

Fadi al-Hassan said the transport “happened in coordination with the army command and in accordance to the administrative, legal and security measures taken at the airport.”

The “captive air training missiles” did not have a warhead, rocket or guidance system, said one source who spoke on condition of anonymity, saying they were headed to the United States after being used to train the Lebanese Armed Forces.

The two were inert air-to-ground Hellfire missiles, the same model as one that arrived in Cuba by mistake in 2014 and was retrieved last month by U.S. officials and representatives from its maker, Lockheed Martin. Cuba said that one had arrived by mistake on a commercial flight from Paris.

In the latest incident, the two inert missiles were discovered in wooden crates by bomb-sniffing dogs at Belgrade airport on Saturday, a source at the Serbian prosecutor’s office said. They arrived from Beirut on Air Serbia and were due to be put on another flight to go to Portland, Oregon.

“Experts are determining whether the missiles were equipped with live or training warheads. … They were packed in proper transportation crates and supplied with paperwork,” the Serbian source said.

However, the first source familiar with the matter described the discovery of the two inert missiles as a “false positive” and said they were never equipped with a warhead.

The Lebanese army also said the Hellfire missiles were training models, without any explosives in them, and that it was returning to the manufacturer.

“They belonged to the Lebanese army, which decided to send them back to the American company that manufactured them upon agreement with it, in accordance with legal and administrative procedures and after training with them had been completed,” the Lebanese army said in a statement carried by the National News Agency.

The AGM 114 Hellfire, made by Lockheed Martin, is an air-to-surface missile that can be used against tanks and other armored vehicles. In addition to a version with a high-explosive warhead, the Hellfire is also produced as a practice weapon.

Air Serbia said it was helping with the investigation and that security and safety were its main priorities.

On Sunday, Lockheed spokesman Joe Stout said the company had seen a media report about the missiles in Serbia but had no further information. Pentagon spokesman Capt. Jeff Davis said the missiles did not belong to the U.S. Department of Defense.